1. Field of the Invention
The invention relates to a method and device for preventing doming phenomena. More specifically, the invention relates to circuitry for preventing doming phenomena in a television receiver.
2. Description of the Related Art
European Patent Application No. EP-A-0,525,976, corresponding to U.S. Pat. No. 5,315,389, describes a prior art intensity control circuit for providing conventional intensity correction and has nothing to do with the prevention of dooming phenomena.
Doming phenomena may occur in case a section of the shadow mask of a cathode ray picture tube is irradiated with high intensity electron beams causing partial distortion of the shadow mask whereby a xe2x80x9cdomexe2x80x9d is formed in the mask. The effect thereof is mis-coloring which is very annoying. Because of the geometry of the tube, the influence of the doming phenomena is rather strong near the periphery of the screen where the electron beam strikes the shadow mask at a relatively large deflection angle.
Electron beams with high intensity are generated in correspondence to a local high luminance signal. To prevent doming phenomena, the luminance signal has to be controlled such that the intensity of the resulting electron beam is limited at least in those regions of the screen where doming is likely to occur.
Circuits which are specifically designed to prevent doming phenomena are already known from the prior art. An example is described in European Patent Application No. EP-A-0,354,518, corresponding to U.S. Pat. No. 4,996,590. The circuit described therein comprises means for processing the luminance signal including a low-pass filter for filtering out only the low frequent components of the luminance signal which, in a first comparator, are compared with a first reference signal related to the horizontal and vertical modulation signals, resulting in a control signal for a charge/discharge circuit which will tend to charge if there are areas on the screen where the luminance will be so high that doming phenomena will appear. The output of this charge/discharge circuit is compared with a second reference level signal resulting into a correction signal, which is used to adapt either the contrast or the luminance of the screen to counteract any doming phenomena.
In this prior art circuit, the first reference signal is generated such that the allowed luminance level is rather large at the center of the screen and is relatively small near the sides of the screen, thereby taking into account that the doming phenomena will appear predominantly near the edges of the screen and not in the center thereof.
It is, inter alia, an object of the invention to provide an improved doming phenomena prevention.
In a method of preventing doming phenomena in accordance with a first aspect of the present invention, luminance histogram distribution values are obtained from a luminance signal, the various luminance histogram distribution values are processed and normalized, the luminance histogram distribution values are processed to correct for at least actual contrast and brightness settings to obtain corrected histogram data, a beam current is predicted from the corrected histogram data, and a correction signal is derived from the predicted beam current and making said correction signal effective in the processing and normalizing step. Preferably, local doming is prevented by determining local histograms to correct local beam currents for at least two windows (32, 34 in FIG. 2).
Advantageously, a doming phenomena preventing circuit in accordance with the present invention, makes predominantly use of a processor performing calculations on luminance data which are already available in the a histogram memory forming part of the normal intensity control circuit. In many cases, a processor is already present and if said processor has a sufficient spare capacity, the doming preventing circuit according to the invention can be realized without adding any substantial hardware to the television receiver circuit. Loading a suitable program into the already present processor to calculate a correction signal from the available data would then solve a major part of the problem.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.